1. Field of the Invention
The present invention relates to a light-emitting element in which an organic compound is provided between a pair of electrodes and the organic compound exhibits luminescence by application of an electric field. Further, the present invention relates to a light-emitting device having such a light-emitting element. Furthermore, the present invention relates to an electronic device and a lighting device having such a light-emitting device.
2. Description of the Related Art
A light-emitting element using an organic compound as a luminous body, which has features such as thinness, lightness, high-speed response, and DC drive at a low voltage, is expected to be applied to a next-generation flat panel display. In particular, a display device in which light-emitting elements are arranged in matrix is considered to have advantages in a wide viewing angle and excellent visibility over a conventional liquid crystal display device.
It is considered that the light emission mechanism of a light-emitting element is as follows: by applying a voltage between a pair of electrodes with an EL layer including a light emitter interposed therebetween, electrons injected from a cathode and holes injected from an anode recombine at a luminescent center of a light-emitting layer in the EL layer to form a molecular exciton, and energy is released to emit light when the molecular exciton relaxes to the ground state. Singlet excitation and triplet excitation are known as excited states, and light emission can probably be achieved through either of the excited states.
In order to improve element characteristics of such a light-emitting element, an improvement of an element structure, a development of a material, and the like have been actively conducted.
For example, there is a report that an electron-injection layer in contact with a cathode is formed by doping an organic compound with a metal having a low work function, such as an alkali metal, an alkaline earth metal, or a rare earth metal to reduce an injection barrier in injection of electrons from the cathode into the electron-injection layer, whereby drive voltage is decreased (e.g., see Patent Document 1).
In relation to this technique, it is also reported that an optical adjustment of an emission spectrum can be conducted without a drive voltage increase (e.g., see Patent Document 2).
Specifically, a light-emitting element is formed to have, between its cathode and EL layer, a layer that is in contact with the cathode and formed by doping a hole-transport organic compound with a metal oxide, and a layer that is in contact with the layer formed by doping the hole-transport organic compound with the metal oxide and formed by doping an electron-transport organic compound with a metal having a low work function, such as an alkali metal, an alkaline earth metal, or a rare earth metal. The layer formed by doping the hole-transport organic compound with the metal oxide has a large thickness to enable the optical adjustment of an emission spectrum. In this case, because the hole-transport organic compound has higher carrier mobility than the electron-transport organic compound, a drive voltage increase can be more suppressed than when the layer formed by doping an electron-transport organic compound with a metal having a low work function has a large thickness.    [Patent Document 1] Japanese Published Patent Application No. H10-270171    [Patent Document 2] Japanese Published Patent Application No. 2005-209643